Derek T. Smith

Effects of ageing and regular aerobic exercise on endothelial fibrinolytic capacity in humans

The capacity of the vascular endothelium locally to release tissue‐type plasminogen activator (t‐PA) is critical for effective endogenous fibrinolysis. We determined the influence of ageing and regular aerobic exercise on the net release of t‐PA across the human forearm in vivo using both cross‐sectional and intervention approaches. First, we studied 62 healthy men aged 22‐35 or 50‐75 years of age who were either sedentary or endurance exercise‐trained. Net endothelial release rates of t‐PA were calculated as the product of the arteriovenous concentration gradient and forearm plasma flow to intra‐arterial bradykinin and sodium nitroprusside. Second, we studied 10 older (60 ± 2 years) healthy sedentary men before and after a 3 month aerobic exercise intervention. Net endothelial t‐PA release was significantly blunted with age in the sedentary men. At the highest dose of bradykinin the increase in t‐PA antigen release was ≈35 % less (P < 0.05) in the older (from −1.0 ± 0.4 to 37.8 ± 3.8 ng (100 ml tissue)−1 min−1) compared with young (from 0.1 ± 0.6 to 56.6 ± 9.2 ng (100 ml tissue)−1 min−1) men. In contrast, the endurance‐trained men did not demonstrate an age‐related decline in the net release of t‐PA antigen. After the exercise intervention, the capacity of the endothelium to release t‐PA increased ≈55 % (P < 0.05) to levels similar to those of the young adults and older endurance‐trained men. Regulated endothelial t‐PA release declines with age in sedentary men. Regular aerobic exercise may not only prevent, but could also reverse the age‐related loss in endothelial fibrinolytic function.

Evidence for agonist‐specific endothelial vasodilator dysfunction with ageing in healthy humans

Endothelium‐dependent vasodilatation declines with advancing age in humans independently of disease. The mechanisms responsible for this decline are not clear. We determined whether the age‐related reduction in endothelium‐dependent vasodilatation in response to acetylcholine reflects a specific agonist‐related defect or rather a more general endothelial cell vasomotor abnormality. Twenty‐two young (23‐35 years) and 41 older (50‐76 years) healthy men were studied. Forearm blood flow (FBF) responses to intra‐arterial infusions of acetylcholine, bradykinin, substance P, isoproterenol (isoprenaline) and sodium nitroprusside were measured by strain‐gauge plethysmography. There were no differences in resting FBF between the young (3.9 ± 0.2 ml (100 ml tissue)−1 min−1) and older men (4.0 ± 0.2 ml (100 ml tissue)−1 min−1). The increase in FBF at the highest dose of acetylcholine was ∼30 % lower (P < 0.01) in the older (from 4.0 ± 0.2 to 12.3 ± 0.7 ml (100 ml tissue)−1 min−1) compared with young men (from 3.9 ± 0.2 to 17.1 ± 1.5 ml (100 ml tissue)−1 min−1). In contrast to acetylcholine, the FBF responses to the other endothelial agonists were not impaired with age. The maximum increases in FBF in response to bradykinin (19.2 ± 1.0 vs. 20.2 ± 0.9 ml (100 ml tissue)−1 min−1), substance P (15.1 ± 0.8 vs. 16.8 ± 0.7 ml (100 ml tissue)−1 min−1) and isoproterenol (17.5 ± 0.9 vs. 17.5 ± 0.9 ml (100 ml tissue)−1 min−1) were not significantly different between the older and young subjects. There were no age‐related differences in the FBF responses to sodium nitroprusside. These results demonstrate that, although acetylcholine‐induced vasodilatation is impaired with age, forearm endothelial vasodilatation in reponse to bradykinin, substance P and isoproterenol are well preserved in healthy men. Moreover, these findings suggest that agonist‐stimulated endothelium‐dependent vasodilatation is not universally impaired with age.

Acute and chronic effects of oestrogen on endothelial tissue-type plasminogen activator release in postmenopausal women

The capacity of vascular endothelium to locally release tissue‐type plasminogen activator (t‐PA) represents an important endogenous defence mechanism against intravascular fibrin deposition and thrombosis. We determined the influence of chronic and acute oestrogen administration on endothelial t‐PA release in postmenopausal women. Sixty‐three healthy postmenopausal women were studied: 31 non‐users (age 58 ± 1 years) and 32 users of hormone replacement therapy, including oestrogen alone (ORT: 62 ± 2 years; n= 15) and in combination with progesterone (HRT: 57 ± 1 years; n= 17). Net endothelial t‐PA release was determined in vivo, in response to intrabrachial infusions of bradykinin and sodium nitroprusside. To examine the acute effects of oestrogen on endothelial t‐PA release, bradykinin and sodium nitroprusside dose‐response curves were repeated in the presence of 17 β‐oestradiol in 20 of the 31 non‐users. Net endothelial release of t‐PA was ≈30 % higher (P < 0.01) in women taking ORT (from 2.0 ± 1.0 to 83.6 ± 9.2 ng (100 ml tissue)−1 min−1) compared with those taking HRT (from 1.4 ± 0.4 to 63.5 ± 5.6 ng (100 ml tissue)−1 min−1) and those not taking supplementation (1.0 ± 0.7 to 63.0 ± 4.7 ng (100 ml tissue)−1 min−1). Intra‐arterial infusion of 17 β‐oestradiol significantly potentiated bradykinin‐induced t‐PA release. Net endothelial release of t‐PA was ≈45 % higher (P < 0.01) after (from 1.0 ± 0.8 to 87.4 ± 9.9 ng (100 ml tissue)−1 min−1) versus before (1.2 ± 0.6 to 60.8 ± 5.6 ng (100 ml tissue)−1 min−1) acute 17 β‐oestradiol administration. Our results suggest that oestrogen has a direct modulatory effect on the capacity of the endothelium to release t‐PA in healthy postmenopausal women. However, progesterone appears to oppose the favourable influence of oestrogen on endothelial fibrinolytic capacity.

Endothelial release of tissue-type plasminogen activator in the human forearm: role of nitric oxide.

Release of tissue-type plasminogen activator (t-PA) from the vascular endothelium is paramount to endogenous thrombolysis potential. In addition to its vasodilator effects, nitric oxide (NO) has important antithrombotic properties, such as inhibition of platelet aggregation. It is currently not clear whether NO influences the capacity of the endothelium to release t-PA. The authors determined whether net endothelial t-PA release is regulated, at least in part, by NO. Endothelial t-PA release was determined, in vivo, in response to intrabrachial infusions of bradykinin (12.5–50.0 ng·100 mL tissue−1·min−1) in the presence and absence of the NO synthase inhibitor, NG-monomethyl-l-arginine (l-NMMA; 5 mg/min) in 12 healthy men. Net release of t-PA across the forearm vascular bed was calculated as the product of arteriovenous concentration gradient and forearm plasma flow. The vasodilator response to bradykinin was significantly blunted (∼30%) with l-NMMA. Although there was no effect of l-NMMA on basal t-PA release, acute release of t-PA to bradykinin was higher (P < 0.01) after (from –0.2 ± 0.5 to 105.2 ± 9.4 ng·100 mL tissue−1·min−1) versus before (from –0.4 ± 0.7 to 48.7 ± 7.3 ng·100 mL tissue−1·min−1) the administration of l-NMMA. Thus, in the absence of NO endothelial t-PA release was enhanced. These results suggest a potential regulatory influence of NO on bradykinin induced endothelial t-PA release.